1. Field of the Invention
The present invention relates to a radio communication apparatus and a radio communication method, and more particularly to a radio communication apparatus which performs frequency hopping by using some frequency channels.
2. Description of the Related Art
The 2.4 GHz band is a radio frequency band in which radio communication systems such as radio communication apparatuses that perform and do not perform the frequency hopping coexist. The frequency hopping is a system for communicating while changing frequency channels to be used at every fixed period (for example, 625 μs) from frequency channels. A device pursuant to the Bluetooth specification (hereinafter, referred to as a Bluetooth device) is a typical radio communication apparatus that performs frequency hopping. In contrast, a station and an access point pursuant to IEEE 802.11b or IEEE 802.11g referred to as a so-called wireless local area network (LAN) are typical radio communication apparatuses that do not perform frequency hopping.
Simultaneous generation of radio signals in identical frequency channels by means of Bluetooth devices, wireless LAN stations and access points cause radio wave interference. As a result, packets are corrupted and the packets are not received at all to cause a communication quality to be degraded. To avoid such radio wave interference, the Bluetooth communication system has a function called adaptive frequency hopping (AFH) to perform the frequency hopping while avoiding the frequency channel now used in the wireless LAN, etc. In the AFH function, a frequency channel assessment is important. A Bluetooth device autonomously assesses a surrounding radio environment and automatically detects a frequency channel with an interference source such as a wireless LAN exists therein.
Two methods of an active assessment system and a passive assessment system as this frequency channel assessment system are known.
In active assessment system, a radio communication device records frequencies of reception errors in packet receiving operations for each frequency channel so as not to use frequency channels apt to cause reception errors. However, active assessment system cannot distinguish between the case in which a device cannot receive packets because the communication partner device sent no signal or is out of a communication range and the case in which the device cannot receive packets because of the radio wave interference. That is, since any case is determined as reception errors, it is hard to accurately distinguish the frequency channels affected with the interference.
In the passive assessment system, a radio communication device uses a timing of non-transmission and non-reception of packets to measure a received signal strength indicator (RSSI) value so as not to use a frequency channel with a large RSSI value. This system is based on the assumption that when other device uses the frequency channel, a large RSSI value will be measured. However, since the passive assessment system utilizes timing of non-transmission and non-reception of the packet, it cannot measure the RSSI value in continuous transmissions and receptions of packets by the other partner device. In the Bluetooth device, a device transmits and receives packets almost continuously for example when performing voice transfer using an HV1 packet type. In this case, the passive assessment system cannot assess the frequency channel. And the passive assessment system needs to operate the transmitter only to measure the RSSI value, and has a problem to increase consumption of power.